Goodwill and Other Intangible Assets (Rollforward of Goodwill by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2018
Goodwill [Line Items]
Goodwill, gross			$ 1,366,800	$ 1,216,400
Goodwill, accumulated impairment loss			(123,800)	(116,000)
Goodwill, net	$ 1,221,440	$ 1,100,400	1,243,034	1,100,400
Goodwill [Roll Forward]
Goodwill, net, beginning balance	1,221,440	1,100,400
Additions from new business combinations	15,100	120,400
Measurement period adjustments, net	6,100	(100)
Currency translation adjustments	400	700
Goodwill, net, ending balance	1,243,034	1,221,440
Communications [Member]
Goodwill [Line Items]
Goodwill, gross			562,100	467,800
Goodwill, accumulated impairment loss			0	0
Goodwill, net	562,100	541,300	562,100	467,800
Goodwill [Roll Forward]
Goodwill, net, beginning balance	541,300	467,800
Additions from new business combinations	14,900	73,200
Measurement period adjustments, net	5,900	300
Currency translation adjustments	0	0
Goodwill, net, ending balance	562,100	541,300
Clean Energy and Infrastructure
Goodwill [Line Items]
Goodwill, gross			152,700	143,100
Goodwill, accumulated impairment loss			0	0
Goodwill, net	152,700	152,600	152,700	143,100
Goodwill [Roll Forward]
Goodwill, net, beginning balance	152,600	143,100
Additions from new business combinations	0	9,500
Measurement period adjustments, net	100	0
Currency translation adjustments	0	0
Goodwill, net, ending balance	152,700	152,600
Oil and Gas [Member]
Goodwill [Line Items]
Goodwill, gross			501,900	455,600
Goodwill, accumulated impairment loss			(123,800)	(116,000)
Goodwill, net	378,100	377,600	378,100	339,600
Goodwill [Roll Forward]
Goodwill, net, beginning balance	377,600	339,600
Additions from new business combinations	0	37,700
Measurement period adjustments, net	100	(400)
Currency translation adjustments	400	700
Goodwill, net, ending balance	378,100	377,600
Electrical Transmission [Member]
Goodwill [Line Items]
Goodwill, gross			150,100	149,900
Goodwill, accumulated impairment loss			0	0
Goodwill, net	150,100	149,900	$ 150,100	$ 149,900
Goodwill [Roll Forward]
Goodwill, net, beginning balance	149,900	149,900
Additions from new business combinations	200	0
Measurement period adjustments, net	0	0
Currency translation adjustments	0	0
Goodwill, net, ending balance	$ 150,100	$ 149,900